She writes music. She likes movies by Stanley Kubrick and John Cassavetes. She yearns for a past she's never experienced. And she's just 20 years old.

"She", of course, is Kristen Stewart and, courtesy of the Twilight movies and heroine Bella Swan, she's arguably the hottest female star on the planet.

It's easy and too simplistic to cast Stewart as sullen, stand-offish and unsmiling. Perhaps the truth behind her oft-reported reluctance to conform to film star stereotypes is that she's fundamentally shy. Yet she has been thrust into a harsh spotlight that she neither anticipated nor welcomed. Not an easy arrangement for a global superstar.

Yet Stewart is not your average screen popsy. If she's rebellious, it's in a low-key manner. Yet she was the actress picked to play Joan Jett, the original American rock chick, in a new biopic of ground-breaking all-girl '70s' band The Runaways.

Undoubtedly, Stewart's box- office appeal has much to do with her casting. Yet there is something within this reserved young woman that is peculiarly retro in mood and feel.

"Meeting Joan for the first time, I was terrified, truly terrified," says Stewart. "If she didn't think that I was right for this, then I didn't think that I was right for it either. So the first meeting could have really gone either way. But I flew up to Washington to see her play and I spent, like, the entire night with her."

It was a New Year's Eve meeting to remember. Immediately afterwards Stewart began work on New Moon, the second film in the Twilight series. She forced herself to hold back, to not obsess over whether she had the-then 50-year-old Jett's vote of approval.

She realised she had the role "when she didn't kick me out of her hotel room," she laughs, adding "we just sort of instantly had a rapport".

Whilst the Twilight movies have handed Stewart an extraordinary celebrity, she is at pains to stress her ordinariness. She wishes to be seen as an actress first; the stardom is a mantle she is forced to wear.

"I watch a lot of '70s' movies," she says by way of explaining her affinity with the legendary rocker.

"I love all of Kubrick's movies. I love old independent movies like Cassavetes movies. Joan was watching funny stuff when she was younger. There's this tape of her when she's 14 so I could hear how she spoke, and it's so different. Now she's very New York and raspy, but back in the day she was light; she was a kid and she just talked about a movie that was funny."

Her relationship with Jett, who executive produced the film, has transcended its making. This odd couple – the awkward, gawky actress and the hard-boiled black-clad queen of proto-punk rock – swap stories about the parallels in their lives.

"She completely understands where I'm coming from. I mean, we're both fairly, unabashedly ourselves, so the advice sort of stops there, do you know what I mean? She says, 'Just be real, just say what you mean,' and there you go."

Notwithstanding the Twilight movies, Stewart's background has been in independent and arthouse pictures, or hybrids of the two. She was excellent as the lovelorn beatnik in Sean Penn's Into the Wild and embraced the opportunity to attack another tasty character in The Runaways.

She may not be entirely convincing but she gave it her best shot. She was fortunate that, as much as she was ostensibly the star attraction, she was also part of a tight ensemble that included Dakota Fanning as the band's vocalist, Cherie Currie.

"It's actually the first movie that I've ever done that wasn't an original, fictional piece of work," says Stewart.

"If we don't capture the essence of The Runaways, then it should be acknowledged. That's really important but it's scary – I now know Joan; I have a relationship with her. Everything would fall away if I thought that I wasn't respecting that."

She adds: "She was a very sensitive girl; the word 'punk' didn't even exist. Everything she wears is kind of light.

"When she gets older, it's armour. She's not trying to be cool, she is trying to be tough, she's compensating, and there's a lot of that. It's an entire lifestyle.

"When the band breaks up, Joan has said to me, 'Well, that's what it feels like to peak at 16'. When it crumbled, it killed her. The Runaways started everything for chicks in rock and roll.

"At the time, it wasn't normal to do what she did. People didn't want to see a girl holding an electric guitar. It was ugly, it just wasn't right. Now it's normal."